1. Field of the Invention
This invention relates to a mask structure for lithography, a method for preparation thereof and a lithographic method by use of the mask structure for lithography, particularly to a mask structure for lithography suitable for projecting a mask pattern on a wafer coated with a photosensitive agent in the steps for production of semiconductors, a method for preparation thereof and a lithographic method by use of the mask structure for lithography.
2. Description of the Prior Art
X-ray lithography has a number of characteristics more excellent than lithography by visible light or UV-rays formerly practiced on the basis of a rectilinear propagation characteristic, incoherence, low diffraction characteristic, etc., and is attracting attention as a powerful means for submicron lithography.
While X-ray lithography has a number of superior points as compared with lithography by visible light or UV-rays, it involves the drawbacks such as insufficient power of the X-ray source, low sensitivity of resist, difficulty in alignment and difficulty in choice of mask material and working method, whereby productivity is low and the cost is high to cause difficulty in practical application.
Among them, referring now to the mask to be used for X-rays lithography, in lithography by visible light or UV-ray, glass plates and quartz plates have been utilized as the mask material holding member (namely light-transmitting member). However, in X-ray lithography, the wavelength of the ray which can be utilized is 1 to 200 .ANG.. The glass plates or quartz plates hitherto used absorbs greatly the ray in this X-ray wavelength region and also the thickness must be made as thick as 1 to 2 mm, whereby X-rays cannot sufficiently be transmitted, and these are unsuitable as the material for the mask material holding member to be used for X-ray lithography.
X-ray transmittance generally depends on the density of a material, and therefore inorganic materials or organic materials with low density are going to be investigated as the material for the mask material holding member to be used for X-ray lithography. Such materials may include, for example, inorganic materials such as simple substances of beryllium (Be), titanium (Ti), silicon (Si) and boron (B), and compounds thereof, or organic compounds such as polyimide, polyamide, polyester, poly-p-xylylene, etc.
For practically using these substances as the material of the mask material holding member to be used for X-ray lithography, they are required to be made into thin films for increasing the amount of X-rays transmitted as much as possible, to a thickness of some microns or less in the case of an inorganic material and some ten microns or less in the case of an organic material. For this reason, in forming, for example, a mask material holding member comprising a thin film of an inorganic material or a composite film thereof, there is proposed the method in which a thin film of Si.sub.3 N.sub.4, SiO.sub.2, BN, SiC, etc. is formed on a silicon wafer, which is excellent in flatness, by vapor deposition, etc. and thereafter the silicon wafer is removed by etching.
On the other hand, as the mask material (namely the X-ray absorbing material) to be used for X-ray lithography held on the mask material holding member as described above, it is preferred to use a thin film of a material having high density such as gold, platinum, tungsten, tantalum, copper, nickel, etc., preferably a thin film with a thickness of 0.5 to 1.mu.. Such a mask can be prepared by, for example, forming a thin film of the above high density material uniformly on the above mask material holding member, then applying a resist thereon, effecting a desired pattern drawing on the resist with an electron beam, light, etc. and thereafter forming to a desired pattern by means of etching, etc.
And, in the X-ray lithography of the prior art as described above, the X-ray transmittance through the mask material holding member was low and hence the mask material holding member is required to be made considerably thin in order to obtain sufficient amount of X-rays transmitted, whereby there is involved the problem that it can be prepared with difficulty.